Various types of digital signal generator are known: British Pat. No. 1,338,633 entitled "Digital frequency generator" describes a generator which comprises an oscillator and a stabilizing arrangement using a linear digital filter defined by finite differential equations.
U.S. Pat. No. 3,905,030 entitled "Digital signal generator" describes a sine-wave signal generator comprising a table of N values of the function y=sin x, together with means for periodically scanning through the table. The frequency f of the signal is related to the frequency F at which the table is scanned and to the scan step-size a by the equation: EQU f.times.N=kF
with k having a maximum possible value of k&lt;N/2 as determined by SHANNON's theorem. The generateur described also includes an adding circuit to make it possible to provide a composite signal of two frequencies. Further, it is known that for digital transmission, signals are encoded according to a logarithmic compression law approximated by straight line segments (C.C.I.T.T. recommendation G711 defining the so-called A law).
It may be assumed that encoding in accordance with the A law is approximately logarithmic, such that a code signal Y representative of a voltage V in millivolts has the form: ##EQU1## This introduces distortion of a kind that is often acceptable. This form can be used to apply a gain G to a signal by mere addition of encoded values: the code signal X corresponding to the amplified signal G.times.V can be written: ##EQU2## However, to add signals, for example to create two-frequency signals from a sine-wave signal generator, it is necessary to linearize these signals by expansion. One known method of doing this consists in using an approximation which makes it possible to linearize using a shift circuit:
Let S be the expanded signal, such that the compressed signal may be written
log.sub.2 S=c+m (where c is the characteristic and m the mantissa of the logarithm with 0&lt;m&lt;1). The approximation consists in replacing 2.sup.m by (1+m), i.e. EQU S=2.sup.c .times.2.sup.m .perspectiveto.2.sup.c .times.(1+m)
Expansion consists in shifting the quantity (1+m) by a number of binary places equal to c.
This approximation causes distortion which may be eliminated by using a look-up table giving values of a parameter p as a function of m such that 2.sup.m =1+p, the table being stored in read-only-memory, for example. This method is used, for example, in the digital frequency generator described in British Pat. No. 1,338,633 mentioned above.
The aim of the invention is to provide a circuit enabling the various multi-frequency signals corresponding to the codes used in a telephone exchange to be created on the basis of signals provided by a digital generator, and to output these signals on digital time slots, for example, in PCM type multiplex connections. Creating and outputting the multi-frequency signals is controlled by a processor, for example, a computer controlling the telephone exchange in the event that the telephone exchange is computer controlled.